(CNN) -- Earth Day may fall later this week, but as far as former NASA astronaut Edgar Mitchell and other UFO enthusiasts are concerned, the real story is happening elsewhere.

Former NASA astronaut Edgar Mitchell, shown in 1998, says "there really is no doubt we are being visited."

Mitchell, who was part of the 1971 Apollo 14 moon mission, asserted Monday that extraterrestrial life exists, and that the truth is being concealed by the U.S. and other governments.

He delivered his remarks during an appearance at the National Press Club following the conclusion of the fifth annual X-Conference, a meeting of UFO activists and researchers studying the possibility of alien life forms.

Mankind has long wondered if we're "alone in the universe. [But] only in our period do we really have evidence. No, we're not alone," Mitchell said.

"Our destiny, in my opinion, and we might as well get started with it, is [to] become a part of the planetary community. ... We should be ready to reach out beyond our planet and beyond our solar system to find out what is really going on out there."

Mitchell grew up in Roswell, New Mexico, which some UFO believers maintain was the site of a UFO crash in 1947. He said residents of his hometown "had been hushed and told not to talk about their experience by military authorities." They had been warned of "dire consequences" if they did so.

But, he claimed, they "didn't want to go to the grave with their story. They wanted to tell somebody reliable. And being a local boy and having been to the moon, they considered me reliable enough to whisper in my ear their particular story."

Roughly 10 years ago, Mitchell claimed, he was finally given an appointment at Pentagon to discuss what he had been told.

An unnamed admiral working for the Joint Chiefs of Staff promised to uncover the truth behind the Roswell story, Mitchell said. The stories of a UFO crash "were confirmed," but the admiral was then denied access when he "tried to get into the inner workings of that process."

The same admiral, Mitchell claimed, now denies the story.

"I urge those who are doubtful: Read the books, read the lore, start to understand what has really been going on. Because there really is no doubt we are being visited," he said.

"The universe that we live in is much more wondrous, exciting, complex and far-reaching than we were ever able to know up to this point in time."

A NASA spokesman denied any cover-up.

"NASA does not track UFOs. NASA is not involved in any sort of cover-up about alien life on this planet or anywhere else -- period," Michael Cabbage said Monday.

Debates have continued about what happened at Roswell. The U.S. Air Force said in 1994 that wreckage recovered there in 1947 was most likely from a balloon-launched classified government project.

Stephen Bassett, head of the Paradigm Research Group (PRG), which hosted the X-Conference, said that the truth about extraterrestrial life is being suppressed because it is politically explosive.

"There is a third rail [in American politics], and that is the UFO question. It is many magnitudes more radioactive than Social Security ever dreamed to be," Bassett said.

it's a pretty f*cking big universe for us to be the only planet capable of "life". whether or not other beings are actually visiting/have visited us I obviously can't say, but I think it's SAFE to assume there's some crazy worlds out there.

it's a pretty f*cking big universe for us to be the only planet capable of "life". whether or not other beings are actually visiting/have visited us I obviously can't say, but I think it's SAFE to assume there's some crazy worlds out there.

oh for sure. you'd be pretty ignorant not to believe that other life didn't exist in the universe but the really interesting is discussion is whether or not we've had contact. i'm a natural skeptic but after reading years and years worth of testimonial from former astronauts, pilots, high ranking military officials, former CIA agents, i can't help but believe there's a massive cover up.

it's a pretty f*cking big universe for us to be the only planet capable of "life". whether or not other beings are actually visiting/have visited us I obviously can't say, but I think it's SAFE to assume there's some crazy worlds out there.

Mathematically, its pretty safe to say there is something that can be defined as life out there. I think.. while its doubtful that aliens have visited us... there are enough strange phenomena that something such as: ghosts,spirits,esp, aliens, strange animals (loch ness sasquatch etc.), is real. I mean the giant squid was supposedly a myth, and so was Troy a century or so ago.

my question is (based off of pictures and cave art and other such things it looks like we used to be in contact with aliens back in time) why did global powers decide to stop this from happening/ and acknowledging it? who were the global powers that decided it and why? or were we "quarentined" and why if so?

oh for sure. you'd be pretty ignorant not to believe that other life didn't exist in the universe but the really interesting is discussion is whether or not we've had contact. i'm a natural skeptic but after reading years and years worth of testimonial from former astronauts, pilots, high ranking military officials, former CIA agents, i can't help but believe there's a massive cover up.

While I believe there is some life out there ,I find it amazingly hard to believe that Earth has been visited by terrestial beings despite the thousads of UFO & alien sightings.

To build a machine capable of visiting from so far away a life form must be extremely intelligent & extremely inquistive/curious to come this far. And to simply do 'fly bys'(as do most UFO's) without making a concerted effort to make contact is implausible.

It would be like Columbus struggling to reach the coast of the Americas & seeing these strange people(Native Americans) ,plants,etc.... & not coming ashore,but simply turning around & going back home.

Why come this far so often not to make any serious contact & exchange of ideas with the inhabitants? People who are that intelligent/inquisitive would never do that.

While I believe there is some life out there ,I find it amazingly hard to believe that Earth has been visited by terrestial beings despite the thousads of UFO & alien sightings.

To build a machine capable of visiting from so far away a life form must be extremely intelligent & extremely inquistive/curious to come this far. And to simply do 'fly bys'(as do most UFO's) without making a concerted effort to make contact is implausible.

It would be like Columbus struggling to reach the coast of the Americas & seeing these strange people(Native Americans) ,plants,etc.... & not coming ashore,but simply turning around & going back home.

Why come this far so often not to make any serious contact & exchange of ideas with the inhabitants? People who are that intelligent/inquisitive would never do that.

who says they haven't maybe they don't talk to "us" but talk to heads of gov't. maybe they don't tell their folks about us either. maybe both civilizations are not ready to know there is more out there. maybe their folks do know about us but realize the "masses" are stupid and would go crazy if we found out.

To build a machine capable of visiting from so far away a life form must be extremely intelligent & extremely inquistive/curious to come this far. And to simply do 'fly bys'(as do most UFO's) without making a concerted effort to make contact is implausible.

The universe is roughly 14,000,000,000 years old, Earth is about 4,000,000,000. Humans have been around for about 100,000 of those 4 billion. Look at how much progress mankind has made in the last 100 years alone! What if another civilization has an extra 1,000 years on us? 10,000? 100,000? A million years? And on and on and on.

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Why come this far so often not to make any serious contact & exchange of ideas with the inhabitants? People who are that intelligent/inquisitive would never do that.

Because they're probably smarter than we are and realize that Earth is full of idiots that would freak out and try to nuke a UFO visitor, resulting in the end of everything on this planet? Just a guess.